Optical device for characterization of a sample

ABSTRACT

The invention relates to an optical device for characterization of a sample ( 3 ) comprising—a source ( 5 ) of parallel or collimated light, having an emission spectrum that is continuous over an observation wavelength range of at least  50  nm width, for illuminating the sample ( 3 ),—a detector ( 7 ) of light scattered by the sample ( 3 ), operating in said observation wavelength range,—a filter ( 9 ) for specific spectral weighting arranged on an optical path upstream of the detector ( 7 ) of the scattered light, and—a processing unit ( 11 ) for processing the measurement signal in order to extract a characterization parameter of the sample ( 3 ).

RELATED APPLICATIONS

This application is the national stage of PCT/FR2016/052352, which wasfiled on Sep. 16, 2016, the contents of which are herein incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to an optical device for characterizing asample or an object. Such a characterization may for example result inthe determination of a surface finish, in particular a roughnessparameter, in particular for a nondestructive analysis of a surface, apolished surface for example.

Such a characterization may also result in the characterization of avolume finish of an object, for example with a view to determining anonuniformity parameter.

Such optical characterizing devices for example allow the surface finishof polished parts to be examined, in particular in the context ofconformity detection and of inspection of the roughness thereof. Thus, aprivileged field of application of the invention is quality control onlines for mass-producing parts or manufacturing parts in high numberwhen a high-quality surface finish is needed and requires the quality ofsuch parts to be checked via an analysis of their surface finish, andtherefore in particular via an analysis of roughness resulting fromdefects in the polishing of polished mechanical parts.

The characterization of a surface is also important for example whencalibrating polishing processes, which are also indispensable if lossesdue to scattering of light in optical components (multilayers, gratings,photonic crystals, etc.) is to be limited. Given the increasingimportance of optronics in industry (in particular telecommunications,measurement methods or the construction of computers based on optroniccomponents) it will be understood that reliable optical characterizingdevices that allow a surface finish of an object to be rapidly measuredare required.

Nonlimitingly, a similar strategic imperative is moreover evident in thefield of fabrics or even the biomedical field (characterization of theskin for example), the textile field the paper-making industry, thesecurity industry, the microelectronics industry or even the cosmeticsindustry.

BACKGROUND AND PRIOR ART

In the prior art, use of a stylus-based roughness tester to determinethe roughness of a surface is known.

However, this type of test takes a relatively long time to carry out,because the roughness must be measured, using a contact technique,segment by segment.

In addition, because of the size of styluses, it is not possible todetect defects of less than a certain size and the smaller the defectsto be detected, the longer the measurements take.

It is also known to use other techniques such as electron, optical ornear-field microscopy, and techniques based on the scatter of light inthe far field.

These techniques allow a rapid inspection of surface finishes, which maybe carried out in a short amount of time.

Among these techniques, those based on the scatter of light in the farfield in particular seem to be advantageous, because they arenonintrusive and because they allow the consequences of scatteringeffects governed by the wave theory of light to be used to extract,almost instantaneously, derived properties of the surface of theexamined part, allowing irregularities such as small scratches or localroughness that would be completely invisible to the naked eye to bedetected.

To do this, the light that an object/sample to be observed scatters inevery direction of space, in reflection or transmission, is firstmeasured, for example using an integrating sphere to take a singlemeasurement of the integrated total scattering in every direction ofspace.

However, such characterization proves to be unsatisfactory, inparticular because it does not take into account the notion of passbandor spatial resolution.

To overcome this difficulty, measuring techniques have been developedthat measure the scattered light emitted by the sample in each directionof space. Angular resolved scattering (ARS) then being spoken of.

However, to obtain a precise result, a scan of scattering angle must beperformed, this requiring a precise mechanical mechanism for moving thedetector of scattered light; not only are such systems complex andexpensive but also the process of analysis is once again quite slow.

Lastly, an integrating hemisphere equipped at its apex with a sensor formeasuring scattered light and with several tens of light sources thatare distributed over the hemisphere and that allow the object/sample tobe analyzed to be illuminated from various angles is known.

In this case, each light source is turned on in turn and an image of thelight scattered by each of the light sources is taken. Therefore, asmany images of scattered light as there are light sources fastened tothe hemisphere are obtained. Although this method is very precise, itrequires a very large volume of data to be dealt with, this making ittoo slow for certain industrial processes.

INVENTION SUMMARY

The present invention aims to at least partially mitigate the drawbacksof the devices of the prior art by providing a device allowing a sampleor an object to be characterized optically (for example its surfacefinish or volume nonuniformity) via one image captured in a singledirection.

To this end, the invention provides an optical device for characterizinga sample comprising

-   -   a source of parallel or collimated light, having a continuous        emission spectrum in an observation wavelength range of at least        50 nm width, for illuminating said sample,    -   a detector of light scattered by the sample, functioning in said        observation wavelength range,    -   a spectral weighting filter placed on the optical path upstream        of the detector of scattered light, and    -   a unit for processing the measurement signal in order to extract        a parameter characterizing the sample, wherein the spectral        weighting filter possesses, in said observation wavelength        range, in reflection or transmission, a spectral filtering        function T(λ) given by the following equation:

T(λ)=η/[λ³Ψ(i ₀,θ₀,λ)F(λ)K(λ)]

-   -   where        -   η is a constant of proportionality,        -   λ is wavelength,        -   θ₀ is the direction of scatter or of observation of the            light,        -   i₀ is the angle of incidence at which the sample is            illuminated by the light source,        -   F(λ) is the spectral power density of the source in the            observation wavelength range,        -   K(λ) is the spectral response of the detector in the            observation wavelength range,        -   Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case of characterization of a            surface of the sample and Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ) in case of            characterization of a volume of the sample,        -   C(i₀,θ₀,λ) is an optical coefficient given by perturbation            theories of the scatter of light by a rough surface, and        -   D(i₀,θ₀,λ) is an optical coefficient this time given by            perturbation theories of the scatter of light by nonuniform            volumes.

The optical characterizing device according to the invention isnoteworthy for the rapidity of the measurements, its compactness and itssimplicity. Specifically, this optical characterizing device functionsvia a single measurement in a single direction of illumination or ofscatter. No scan, neither in wavelength nor spatial or angular isrequired.

The weighting filter thus makes it possible to make it so that themeasurement of scatter in a single direction and under white light is,in the surface-characterizing mode, proportional to the roughness of theilluminated surface and, in the volume-characterizing mode, proportionalto the nonuniformity of the medium from which the sample or object iscomposed.

This is very advantageous in particular when it is desired tocharacterize samples that are difficult or even impossible to accessand/or manipulate, or when these samples can be illuminated or measuredonly in certain particular directions.

These observations apply both to objects or samples to be characterizedin the laboratory or center of studies, but also to long-distancecharacterization such as may be the case with satellites or otheron-board systems.

The detecting device according to the invention may have one or more ofthe following features, which may be implemented alone or incombination:

According to one aspect, the light source may be chosen from thefollowing group: an incandescent lamp, a white-light laser, awhite-light diode or a non-filtered supercontinuum laser.

According to another aspect, the detector of scattered light comprises aCCD sensor or a CMOS sensor.

The spectral weighting filter is for example placed between the sampleand the detector of scattered light, or indeed between the light sourceand the sample.

The weighting filter for example comprises a stack of thin films.

The stack of thin films may be deposited using technologies fordepositing semiconductor films or thin optical films.

The stack of thin films for example comprises at least one firstmaterial having a first refractive index and one second material havinga second refractive index that is different from the first refractiveindex, said materials being placed in alternation.

According to one example embodiment, the difference between the firstand second refractive index is larger than 0.4.

The films possess, by way of example, a thickness smaller than 350 nmand in particular comprised between 20 nm and 325 nm. More generally,the thickness of the films has the order of magnitude a fraction of theaverage wavelength of the light source.

According to one particular embodiment, the first material is silica andthe second material is tantalum oxide.

More generally, the filters may be produced with any number ofdielectrics, such as oxides, nitrides, sulfides, etc., and may eveninclude thin metal or semiconductor films. According to yet anotheraspect, in the presence of the spectral weighting filter configured forsurface scattering, the processing unit is programmed to extract theroughness δ of the surface of the sample using the following formula:

δ²=[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection,        -   η is the constant of proportionality precedingly associated            with the weighting filter, and        -   v(θ₀) is the signal delivered by the detector of scatter in            the direction θ₀ when the sample is illuminated with the            light source.

According to yet another aspect, in the presence of a spectral weightingfilter configured for volume scattering, the processing unit isprogrammed to extract a normalized standard deviation of thenonuniformity (Δn/n) of the volume of the sample using the followingformula

(Δn/n)²=(1/4)[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection, and        -   η is the constant of proportionality precedingly associated            with the weighting filter.

In this case, v(θ₀) is the signal delivered by the detector of scatterin the direction θ₀ when the sample is illuminated with the lightsource.

Furthermore, the measurement v(θ₀) is this time a measurement of volumescattering of light.

The invention also relates to a method for characterizing a samplecomprising the following steps:

-   -   a sample is illuminated with a source of parallel or collimated        light having a continuous emission spectrum in an observation        wavelength range of at least 50 nm width,    -   light scattered by the sample is detected, a spectral filter for        weighting the light source being placed on the optical path        upstream of the detector of scattered light, and    -   a parameter characterizing the sample is extracted,    -   wherein that the spectral weighting filter possesses, in said        observation wavelength range, in reflection or transmission, a        spectral filtering function T(λ) given by the following        equation:

T(λ)=η/[λ³Ψ(i ₀,θ₀,λ)F(λ)K(λ)]

-   -   where        -   η is a constant of proportionality,        -   λ is wavelength,        -   θ₀ is the direction of scatter of the light,        -   i₀ is the angle of incidence at which the sample is            illuminated by the light source,        -   F(λ) is the spectral power density of the source in the            observation wavelength range,        -   K(λ) is the spectral response of the detector in the            observation wavelength range,        -   Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case of characterization of a            surface of the sample and Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ) in case of            characterization of a volume of the sample,        -   C(i₀,θ₀,λ) is an optical coefficient given by perturbation            theories of the scatter of light by a rough surface, and        -   D(i₀,θ₀,λ) is an optical coefficient this time given by            perturbation theories of the scatter of light by nonuniform            volumes.

The method according to the invention may have one or more of thefollowing features, which may be implemented alone or in combination:

According to one aspect, in the presence of the weighting filterconfigured for surface scattering, the roughness δ of the surface of thesample is extracted using the following formula:

δ²=[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection,        -   η is the constant of proportionality precedingly associated            with the weighting filter, and        -   v(θ₀) is the signal delivered by the detector of scatter in            the direction θ₀ when the sample is illuminated with the            light source.

According to yet another aspect, in the presence of a spectral weightingfilter configured for volume scattering, a normalized standard deviationof the nonuniformity (Δn/n) of the volume of the sample is extractedusing the following formula:

(Δn/n)²=(1/4)[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection,        -   η is the constant of proportionality precedingly associated            with the weighting filter, and        -   v(θ₀) is the signal delivered by the detector of scatter in            the direction θ₀ when the sample is illuminated with the            light source.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Other advantages and features will become apparent on reading thedescription of the invention, and from the following figures, in which:

FIG. 1 shows a schematic of a detecting device according to theinvention,

FIG. 2 shows a graph as a function of wavelength of the spectralvariations of an example of a light source and the spectral variationsof a detector of scattered light and the product thereof,

FIG. 3 shows a graph as a function of wavelength of the spectralvariation in a weighting function,

FIG. 4 shows an example of a weighting filter for the illuminating anddetecting device of FIG. 1,

FIG. 5 (which is composed of FIGS. 5-1, 5-2 and 5-3) reproduces in atable the structural features of one embodiment of a weighting filter,and

FIG. 6 shows the various steps of the method according to the invention.

DETAILED DESCRIPTION

In the figures, elements that are the same have been referenced with thesame references.

The following embodiments are given by way of nonlimiting example.Although the description refers to one or more embodiments, this doesnot necessarily mean that each reference relates to the same embodiment,or that the features apply to only one single embodiment. Singlefeatures of various embodiments may also be combined to create otherembodiments.

In the description, certain elements or parameters may be indexed, suchas for example first element or second element and first parameter andsecond parameter or even first criterion and second criterion, etc. Inthis case, it is a question of simple indexation to differentiate anddenote elements or parameters or criteria that are similar but notidentical. This indexation does not imply a priority of one element,parameter or criterion with respect to another and such denominationsmay easily be interchanged without departing from the scope of thepresent description.

In the present description, by “spectral weighting filter”, what ismeant is an optical device that modifies the spectral distribution ofincident radiation, such as for example an interference filter,micro-mirrors or spectrometers.

FIG. 1 schematically shows one example embodiment of an optical device 1for characterizing a sample 3 or an object.

The words sample and object (object of study) are here used as synonyms.The sample may be planar or have a certain shape.

The object or sample may for example be a glass, metal or semiconductorsubstrate. It may also be an organic substrate, or even a plant orbiological tissue. These objects or samples may be related to optical,mechanical, textile or paper-making applications. The objects may alsobe liquid or gaseous.

This optical device 1 for characterizing a sample 3 comprises, forilluminating the sample 3, a source 5 of parallel light having acontinuous emission spectrum in an observation wavelength range of atleast 50 nm, in particular 100 nm or even in all the visible spectrumbetween 400 nm and 750 nm and/or a portion of the IR spectrum.

More generally, the spectrum of the source must be extensive, incontrast to the spectrum of a monochromatic laser. Therefore, the sourcewill be said to be “white”, and it will possibly also consist of any“broad” spectrum source, including white lasers or diodes, etc.

By default, the adjective “broad” will possibly be considered to meanthat the spectral width is comprised between 50 nm and 1000 nm, it beingunderstood that the performance of the instrument will improve with thisspectral width. It will here be noted that the spectral width of thesource, associated with the measurement directions, will set the spatialresolution and the frequency window for the roughness measurement.

Typically, the illuminated size of the sample 3 may be comprised between1 mm² and 4 cm².

Of course, in the case of surface characterization to obtain a roughnessδ, the sample 3 must have a good average planarity.

Specifically, for example for a substrate, the planarity must be suchthat the angle of incidence of local illumination must not vary by morethan a few degrees.

The field of application in terms of surface roughness or volumenonuniformity is defined by that of perturbation theories (this will beexplained below); this means that the scattering must remain small withrespect to the incident flux, or even that the roughness must remainsmall with respect to the wavelength of the radiation.

In the case of volume scattering, the nonuniformity must be small withrespect to 1.

When these conditions are not met, measurement precision is lost.

Thus, optical surfaces (polished surfaces, or surfaces generating asmall amount of scattered flux) lie fully within the field ofapplication of the present invention, as do weakly nonuniform volumes.

By way of example, the light source 5 of continuous emission spectrumfor example comprises an incandescent lamp, a white-light laser, awhite-light diode or indeed a non-filtered supercontinuum laser.

The light source 5 may be continuous-wave or pulsed. In the case of apulsed light source 5, the calculations are carried out differently inorder to take into account the pulsed aspect of the source, and temporalanalysis of the scattered signal then returns the roughness ornonuniformity values, by virtue of a time/frequency analogy.

The optical characterizing device 1 in addition comprises a detector 7of the light scattered by the sample, such as for example a CCD sensoror a CMOS sensor (such as found in a video camera). Of course, otheroptical detectors, such as for example a photomultiplier, may be usedprovided that they are suitable for detecting the scattered light.

As may be seen in FIG. 1, the light source 5 emits along an axis havingan angle of incidence i₀ of illumination with respect to theperpendicular to the surface of the sample 3 and the detector 7 detectslight scattered at an observation angle θ₀.

The variation in the angle of incidence of illumination has the effectof moving the passband of the measurement without decreasing thevalidity of the method.

It will be recalled that the spatial-frequency passband lies between thevalues n₀|sin(θ₀)−sin(i₀)|/λ₂ and n₀|sin(θ₀)−sin(i₀)|/λ₁, where λ₁ andλ₂ are the minimum and maximum wavelengths of the observation wavelengthrange of the source. Generally, the roughness measurement remains fullyvalid in so far as the passband is specified.

A spectral filter 9 for weighting the light emitted by the source 5 isplaced on the optical path upstream of the detector 7 of scatteredlight, and more specifically between the sample 3 and the detector 7. Ofcourse, this spectral weighting filter 9 may also be placed between thelight source 5 and the sample 3.

The weighting filter 9 possesses, in said observation wavelength range,in reflection or transmission, a spectral filtering function T(λ) givenby the following equation:

T(λ)=η/[λ³Ψ(i ₀,θ₀,λ)F(λ)K(λ)]  (eq. 1)

-   -   where        -   η is a constant of proportionality,        -   λ is wavelength,        -   θ₀ is the direction of scatter of the light,        -   i₀ is the angle of incidence at which the sample is            illuminated by the light source,        -   F(λ) is the spectral power density of the source in the            observation wavelength range,        -   K(λ) is the spectral response of the detector in the            observation wavelength range,        -   Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case of characterization of a            surface of the sample and Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ) in case of            characterization of a volume of the sample (3),        -   C(i₀,θ₀,λ) is an optical coefficient given by perturbation            theories of the scatter of light by a rough surface, and        -   D(i₀,θ₀,λ) is an optical coefficient this time given by            perturbation theories of scatter by nonuniform volumes.

The spectral weighting filter 9 therefore possesses, in reflection ortransmission, a spectral filtering function T(λ) that is proportional tothe inverse of the quantity given by the cube of the wavelength λmultiplied by the product of three terms that are the spectral responseK(λ) of the detector, the spectral power density F(λ) of the source andan optical coefficient Ψ(i₀,θ₀,λ).

Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case of characterization of a surface of thesample and C(i₀,θ₀,λ) is given by perturbation theories of scattering bya rough surface (ref: Claude Amra, C. Grèzes-Besset, and L. Bruel,“Comparison of surface and bulk scattering in optical multilayers”,Appl. Opt. 32, 5492-5503 (1993)).

Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ) in case of characterization of a volume of thesample, and D(i₀,θ₀,λ) is given by perturbation theories of scatteringby volumes (ref: Claude Amra, C. Grèzes-Besset, and L. Bruel,“Comparison of surface and bulk scattering in optical multilayers”,Appl. Opt. 32, 5492-5503 (1993)).

It will be noted here that an approximate formula may be used for thisfilter in the case of surface scattering, assuming that the coefficientC(i₀,λ) does not depend on the direction of scatter; in this case, thefilter becomes proportional to the ratio λ/(FK).

The detector 7 of scattered light is connected to a unit 11 forprocessing the measurement signal in order to extract the roughness ofthe surface or the nonuniformity of the sample 3, as will be describedin more detail below.

As will be clear from the above equation (eq. 1), the spectral weightingfilter is manufactured specifically for one light source 5 and onedetector 7 of scattered light.

Specifically, equation (1) takes into account the spectral power densityF(λ) of the light source 5 in the observation range and the spectralresponse K(λ) of the detector 7 in the observation wavelength range.

FIG. 2 shows a graph as a function of wavelength λ of the relativespectral variations F(λ) of an example of a light source (curve 100) andthose K(λ) of a detector (curve 102) of scattered light and the productthereof F(λ)*K(λ) (curve 104).

These data may generally be obtained from the datasheets ofmanufacturers of optical equipment.

In the example of FIG. 2, the light source 5 is a supercontinuum laserand the detector 7 is a CCD sensor.

On the basis of these data, it is possible, by virtue of equation 1, todefine a theoretical curve of reflectance or transmittance T(λ), whichis drawn in FIG. 3 (curve 106) with a thin line for all the domain 400nm-1000 nm.

On the basis of such a theoretical curve, it is possible to design bycalculation a spectral weighting filter 9 formed for example of a stackof thin planar films. Synthesis techniques for performing thiscalculation are well-known (reference: H. A. Macleod, A. MacLeod, ThinFilm Optical Filters, ISBN-13:9780750306881, 3rd edition, January 2001,Taylor and Francis, Inc.).

An example of the ten first films of a spectral weighting filter 9 isshown in FIG. 4, the entirety of the features of an example weightingfilter of 98 films being reproduced in FIG. 5 (composed of FIGS. 5-1,5-2 and 5-3) in the form of a table detailing, in the first column, anorder No. “#” of the films (C_(j), j being an integer number), in thesecond column, physical thickness, in the third column, opticalthickness, in the fourth column, the letter “H” designates a material ofhigh refractive index and the letter “B” a low refractive index, and, inthe fifth column, the material from which the film in question of No.“#” is made (in the present case Ta₂O₅ and SiO₂), the weighting filterbeing passed through by light scattered from the last film (No. 98)toward the first film. It will however be noted that for these linearand nonabsorbent filters, the transmittance does not depend on thedirection of propagation of the light.

This spectral weighting filter 9 is formed from a stack of thin planarand parallel films, in the present example the ten first of which arereferenced C₁₀-C₁₀ in FIG. 4.

With regard to the manufacture of such a thin-film filter, the readermay for example refer to [P. W. Baumeister, Optical Coating Technology,ISBN: 9780819453136, SPIE Press Book (2004)]. The films C, are forexample deposited one after the other using thin-film depositiontechnology such as physical vapor deposition (PVD). Of course, othertechnologies specific to thin optical films such as for exampleevaporation or ion-assisted evaporation, ion-beam sputtering, magnetronsputtering and sol-gel processing are envisionable.

In the present case, the stack of thin films comprises at least onefirst material having a first so-called low refractive index (letter “B”in the table of FIG. 5), silica for example (average refractive index inthe visible n_(SiO2)≈1.487) and a second so-called high index material(letter “H” in the table of FIG. 5), tantalum oxide for example(refractive index varying approximately about n_(Ti2O5)≈2.15 in thevisible), the refractive index of which is different from the firstrefractive index, said materials being placed in alternation one abovethe other.

The difference between the first and second refractive index is inparticular larger than or equal to 0.4, and here even larger than 0.5,and the films possess a thickness smaller than 350 nm, and in particularcomprised between 20 nm and 325 nm. The larger the difference betweenthe first and second refractive index, the smaller the number of films.

In the case where the index difference is small, by increasing thenumber of thin films, it is possible to obtain similar performancelevels. However, it will be understood that the cost of a weightingfilter 9 depends on the number of films that make it up. Therefore, itis desirable for the refractive index difference to be as large aspossible in order to be able to decrease the number of films. Havingsaid that, techniques for synthesizing multilayer stacks sometimesrequire many films of materials of little-different indices to be used,in particular to remove or decrease potential oscillations in thespectral profile.

Moreover, the order of magnitude of the thicknesses is generally afraction of the average wavelength in the observation wavelength rangeof the illuminating source.

According to one variant, it is also possible to manufacture filterswith more than two different materials, each having its own refractiveindex, without departing from the scope of the present invention.

FIG. 5 shows, in the form of a table, the structure of a weightingfilter 9 and indicates, for each of the 98 films that make up the filter9, the thickness of the film in nm and the material of the film inquestion.

With these features, a weighting filter 9 that possesses a filteringfunction T(λ) meeting the specifications required for the scattering tobe proportional to roughness is obtained. This is shown in FIG. 3 withthe curve 108 (circles or thick line), which shows the transmittance Tof the weighting filter 9 in the table of FIG. 5 as a function ofwavelength λ.

It may be seen that, in the observation range, which is in the presentcase comprised between 400 nm and 900 nm, a very good concordancebetween the curve 106 representing the theoretical transmission curve asexplained above and the curve 108 is obtained.

As already detailed above, the weighting filter 9 will need to berecalculated in the general case, depending on calibration features ofthe system (spectral power of the source, spectral response of thedetector, etc.) and for a given spectral window. In any case, stacks ofthin films allow the sought-after filter to be produced.

To characterize the surface of a sample, the processing unit 11 forexample comprises a computer with a processor and memories, and isprogrammed to extract the roughness δ of the surface of the sample δusing the following formula:

δ²=[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]  (eq. 2)

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection,        -   η is the constant of proportionality precedingly associated            with the weighting filter, and        -   v(θ₀) is the scattering signal (surface scattering)            delivered in the direction θ₀ when the sample is illuminated            with a source of white light.

It will therefore be understood that the roughness δ of the sample 3 maybe analysed via a single measurement (i.e. without a spatial or spectralscan) without loss of precision. It will therefore be understood thatthe characterizing device according to the invention allows asubstantial amount of time to be saved while delivering reliable andprecise measurement results.

To characterize a volume of a sample 3, the processing unit 11 forexample comprises a computer with a processor and memories, and isprogrammed to extract the volume nonuniformity (Δn/n) of the sampleusing the following formula:

(Δn/n)²=(1/4)[v(θ₀)/ηΔΩ][2π(n ₀ sin θ₀)²]  (eq. 3)

-   -   where        -   θ₀ is the angle of the detection direction,        -   n₀ is the optical refractive index of the medium in which            the measurement is carried out (in general air, so n₀≈1),        -   ΔΩ is the measurement solid angle of the detection,        -   η is the constant of proportionality precedingly associated            with the weighting filter, and        -   v(θ₀) is the scattering signal (volume scattering) delivered            in the direction θ₀ when the sample is illuminated with a            source of white light.

It will therefore be understood that the volume nonuniformity (Δn/n) ofthe sample 3 may be analyzed via a single measurement (i.e. without aspatial or spectral scan) without loss of precision. It will thereforebe understood that the characterizing device according to the inventionallows a substantial amount of time to be saved while deliveringreliable and precise measurement results.

The difference between equations 2 and 3 (eq. 2 and eq. 3) resides inthe fact that the spectral weighting filter 9 must be adapted to thestudied scattering case, as already mentioned and explained.

Equations 2 and 3 (eq. 2 and eq. 3) may be derived in the following way:

1) Surface Case (Surface Roughness)

Electromagnetic perturbation theories state that the monochromaticintensity scattered in one direction of space may be written, in theplane of incidence (ϕ=0°) as follows:

I(i ₀,θ₀,λ)=C(i ₀,θ₀,λ)γ(θ₀,λ)   (i)

This relationship (i) is given for non-polarized light, for reasons ofsimplicity of the description. i₀ and θ₀ are the directions ofillumination and of scatter, and λ is wavelength. The light source 5 istherefore here deemed to emit at a single wavelength. In the case ofpolarized light, the formulae are very similar with a slightly modifiedcoefficient C.

Let us now consider the case where a light source 5 with a broadwavelength spectrum, i.e. a so-called white light source, having anemission spectrum that is continuous in an observation wavelength rangeof at least 50 nm width is used to illuminate said sample 3, with aspectral power density given by F (λ).

With an illumination produced by this light source 5, the scatteringsignal v measured in a direction ϑ₀ is an integral over all of thewavelengths, i.e.:

v(θ₀)=ΔΩ∫_(λ) C(i ₀,θ₀,λ)γ(θ₀,λ)F(λ)K(λ)dλ  (ii)

-   -   where ΔΩ is the measurement solid angle, K(λ) is the spectral        response of the detector, and C(i₀,θ₀,λ) is an optical        coefficient given by surface perturbation theory.

It will be noted that the coefficient C(i₀,θ₀,λ) does not depend onsurface roughness, but only on the conditions of illumination and ofobservation, and on the refractive indices of the media (ref: ClaudeAmra, C. Grèzes-Besset, and L. Bruel, “Comparison of surface and bulkscattering in optical multilayers”, Appl. Opt. 32, 5492-5503 (1993)). Itwill be noted here that if a weighting filter T(λ) is inserted upstreamof the detector or after the source, the signal given by (ii) becomes:

v(θ₀)=ΔΩ∫_(λ) C(i ₀,θ₀,λ)γ(θ₀,λ)F(λ)K(λ)T(λ)dλ  (iii)

At this stage, it is desired that the measured signal be proportional tothe roughness δ of the sample 3, which is given by:

δ²=(1/Σ)∫_(x,y) h ²(x,y)dx dy   (iv)

-   -   where h(x,y) describes the topography of the surface to be        measured, and Σ the illuminated or explored area. Furthermore,        in the Fourier plane this roughness may be written:

δ²=∫_(v)γ(v)dv=∫ _(v,ϕ)γ(v,ϕ)vdvdϕ=2π∫_(v) vγ*(v)dv   (v)

-   -   where:        -   γ(v)=(1/Σ)|ĥ(v)|² (vi) is the roughness spectrum of the            surface        -   ĥ(v) is the Fourier transform of the surface        -   v=(v_(x),v_(y))=v(cos ϕ,sin ϕ) (vii) is the conjugate            spatial frequency in (x,y)        -   v=|v| is the modulus of the spatial frequency        -   γ*(v)=(1/2π)∫_(ϕ)γ(v,ϕ)dϕ (viii) is the azimuth or polar            average of the roughness spectrum        -   ϕ is the direction of the spatial frequency in the            (v_(x),v_(y)) plane.

It is also known that the modulus of the spatial frequency is related tothe normal scattering angle θ₀ by:

v=n ₀ sin θ₀/λ  (ix)

It is now necessary to compare formulae (iii) and (v). To do this, achange of variable is first made in equation (v), using equation (ix)with λ as the new variable in the integral. The following is thusobtained:

δ²=2π(n ₀ sin θ₀)²∫_(λ)γ(θ₀,λ)dλ/λ ³   (x)

By comparing the integral of equation (x) to the integral of equation(iii), it may be seen that these integrals are identical if theweighting filter respects:

C(θ₀,λ)F(λ)K(λ)T(λ)=η/λ³   (xi)

-   -   where η is a constant of proportionality. Therefore:

T(λ)=η/[λ³ C(θ₀,λ)F(λ)K(λ)]  (xii)

Thus, with such a spectral weighting filter 9, the signal measured underwhite light in a given direction is proportional to the roughness of thesurface, which is therefore easily measured after suitable calibration.

The fact of modifying the angle of incidence or of illumination does notmodify the method, but simply moves the measuring passband, i.e. theresolution with which the roughness is measured; it is therefore adegree of freedom that is an additional advantage and that allows, whereappropriate, resolution to be adjusted.

It will however be noted that this is true only for an isotropicsurface, for which the spectrum is independent of polar angle, namelywhen:

γ*(v)=γ(v)=γ(v)   (xiii)

If the surface is not isotropic, it is enough, for illumination ofnormal incidence and under non-polarized light, to iterate thescattering measurement while making the sample rotate about its normal,and to then average the measurements. This works because the Fouriertransform preserves rotations. In other words, making the sample rotateabout its normal also makes the spatial frequency of the same anglerotate.

2) Volume Case (volume Nonuniformity)

The method is analogous in the volume case, the differences being:

-   -   a) A coefficient D(i₀,θ₀,λ) (denoted C(i₀,θ₀,λ) for the surface        case above) which is given by perturbation theories of        scattering by nonuniform volumes [ref: Claude Amra, C.        Grèzes-Besset, and L. Bruel, “Comparison of surface and bulk        scattering in optical multilayers”, Appl. Opt. 32, 5492-5503        (1993)]. It will be noted here that the random variations in the        index of the volume must be uniquely transverse (in x, y), and        that the depth variation (along z) of the index may be        exponential.    -   b) A permittivity spectrum that must replace the roughness        spectrum. This amounts to replacing the Fourier transform of the        profile of the surface (case for roughness) with the Fourier        transform of the normalized variations p(x, y)=Δε/ε in the        permittivity ε in the volume. The normalized standard deviation        of the nonuniformity is then given by:

|Δn/n| ²

=(1/4)∫_(v)γ(v)dv   (xiv)

-   -   with the permittivity spectrum:

γ(v)=(1/Σ)|{circumflex over (p)}(v)|²   (xv)

The optical characterizing device 1 according to the invention isnoteworthy for the rapidity of the measurements, its compactness and itssimplicity. Specifically, this optical characterizing device functionsvia a single measurement in a single direction of illumination or ofscatter. No scan, neither in wavelength nor spatial or angular isrequired.

The device 1 for characterizing a sample 3 may function in the followingway:

-   -   In a step 200, the sample 3 is illuminated with the source 5 of        parallel or collimated light having a continuous emission        spectrum in an observation wavelength range of at least 50 nm        width.    -   Next, in step 202, light scattered by the sample 3 is detected.        A spectral weighting filter 9 having the features described        above (see eq. 1) is placed on the optical path upstream of the        detector 7 of scattered light.    -   In a step 204, a parameter characterizing the sample 3 is        extracted.    -   In the presence of a spectral weighting filter 9 configured for        surface scattering, the roughness δ of the surface of the sample        3 is extracted using equation 2 (eq. 2).    -   In the presence of a spectral weighting filter 9 configured for        volume scattering, a normalized standard deviation of the        nonuniformity (Δn/n) of the volume of the sample is extracted        using equation 3 (eq. 3).

1-15. (canceled)
 16. An apparatus comprising an optical device forcharacterizing a sample, said optical device comprising a source, adetector, a filter, and a unit, wherein said source is a source ofparallel or collimated light for illuminating said sample, wherein saidsource has a continuous emission spectrum in an observation wavelengthrange of that has a width of at least fifty nanometers, wherein saiddetector is a detector of light scattered by said sample, wherein saiddetector functions in said observation wavelength range, wherein saidfilter is a spectral weighting filter that is placed on an optical pathupstream of said detector, and wherein said unit is configured forprocessing a measurement signal to extract a parameter thatcharacterizes said sample, wherein said filter has, within saidobservation wavelength range, in reflection or transmission, a spectralfiltering function T(λ) given by η/[λ³Ψ(i₂,θ₀,λ)F(λ)K(λ)], wherein η isa constant of proportionality, wherein λ is wavelength, wherein θ₀ is adirection in which said light is scattered, wherein i₀ is an angle ofincidence at which said source illuminates said sample, wherein F(λ) isa spectral power density of said source in said observation wavelengthrange, K(λ) is a spectral response of said detector in the observationwavelength range, wherein Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case ofcharacterization of a surface of said sample and Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ)in case of characterization of a volume of said sample, whereinC(i₀,θ₀,λ) is an optical coefficient given by perturbation theories oflight scattering by a rough surface, and wherein D(i₀,θ₀,λ) is anoptical coefficient this time given by perturbation theories of lightscattering by non-uniform volumes.
 17. The apparatus of claim 16,wherein said source is selected from the group consisting of anincandescent lamp, a white-light laser, a white-light diode and anon-filtered supercontinuum laser.
 18. The apparatus of claim 16,wherein said detector comprises a sensor selected from the groupconsisting of a CCD sensor and a CMOS sensor.
 19. The apparatus of claim16, wherein said filter is between said sample and said detector, 20.The apparatus of claim 16, wherein said filter is between said sourceand said sample.
 21. The apparatus of claim 16, wherein said filtercomprises a stack of thin films.
 22. The apparatus of claim 21, whereinsaid the stack is a stack that has been deposited in the manner in whichsemiconductor films or thin optical films are deposited.
 23. Theapparatus of claim 21, wherein said stack comprises a first materialhaving a first refractive index and a second material having a secondrefractive index, wherein said second refractive index differs from saidfirst refractive index, and wherein said first and second materials formalternate films in said stack.
 24. The apparatus of claim 23, whereinsaid first and second refractive indices differ by at least 0.4.
 25. Theapparatus of claim 23, wherein said films are less than 350 nanometersthick.
 26. The apparatus of claim 23, wherein said films are between 20and 325 nanometers thick.
 27. The apparatus of claim 23, wherein saidfirst material is silicon dioxide and wherein said second material istantalum pentoxide.
 28. The apparatus of claim 16, wherein said filteris configured for surface scattering, wherein said processing unit isprogrammed to extract a roughness d of said surface using a formulagiven by δ²[v(θ₀)/ηΔΩ][2π(n₉ sin θ₀)²], wherein θ₀ is an angle ofdetection direction, wherein n₀ is an optical refractive index of amedium in which said measurement is carried out, wherein ΔΩ is ameasurement solid angle of detection, wherein η is a constant ofproportionality associated with said filter, and wherein v(θ₀) is asignal delivered by a detector of scatter in a direction θ₀ when saidsource illuminates said sample.
 29. The apparatus of claim 16, whereinsaid filter is configured for volume scattering, wherein said processingunit is programmed to extract a normalized standard deviation of anonuniformity of a volume of said sample (Δn/n) using a formula given by(Δn/n)²=(1/4)[v(θ₀)/ηΔΩ][2π(n₀ sin θ₀)²], wherein θ₀ is an angle ofdetection direction, wherein n₀ is the optical refractive index of themedium in which said measurement is carried out, wherein ΔΩ is ameasurement solid angle of detection, wherein η is a constant ofproportionality, and wherein v(θ₀) is a signal delivered by saiddetector in said angle of detection direction θ₀ when said sourceilluminates said sample.
 30. A method comprising characterizing asample, wherein characterizing said sample comprises illuminating saidsample with a source, detecting light scattered by said sample, andextracting a parameter characterizing said sample, wherein said sourceis a source of parallel or collimated light having a continuous emissionspectrum in an observation wavelength range of at least fifty nanometersin width, wherein detecting said light comprises placing a filter on anoptical path upstream of a detector of scattered light, wherein saidfilter is a spectral weighting filter that has, in said observationwavelength range, in reflection or transmission, a spectral filteringfunction T(λ) given by T(λ)=η/[λ³Ψ(i₀,θ₀,λ)F(λ)K(λ)], wherein η is aconstant of proportionality, wherein λ is wavelength, wherein θ₀ is adirection of scattered light, wherein i₀ is an angle of incidence atwhich source illuminates said sample, wherein F(λ) is a spectral powerdensity of said source in said observation wavelength range, whereinK(λ) is a spectral response of said detector in said observationwavelength range, wherein Ψ(i₀,θ₀,λ)=C(i₀,θ₀,λ) in case ofcharacterization of a surface of said sample and Ψ(i₀,θ₀,λ)=D(i₀,θ₀,λ)in case of characterization of a volume of said sample, whereinC(i₀,θ₀,λ) is an optical coefficient given by perturbation theories ofthe scatter of light by a rough surface, and wherein D(i₀,θ₀,λ) is anoptical coefficient time given by perturbation theories of scatter bynonuniform volumes.
 31. The method of claim 30, wherein said filter isconfigured for surface scattering, wherein said parameter is a roughnessof said surface of said sample, and wherein extracting said parametercomprises extracting said parameter using a formula given byδ²=[v(θ₀)/ηΔΩ][2π(n₀ sin θ₂)²], wherein θ₀ is an angle of detectiondirection, n₀ is an optical refractive index of a medium in which saidmeasurement is carried out, ΔΩ is a measurement solid angle of saiddetection, η is a constant of proportionality associated with saidfilter, and v(θ₀) is a signal delivered by said detector in a directionθ₀ when said source illuminates said sample.
 32. The method of claim 30,further comprising configuring said filter for volume scattering,wherein extracting comprises extracting a normalized standard deviationof a nonuniformity (Δn/n) of a volume of said sample using a formulagiven by (Δn/n)²=(1/4)[v(θ₀)/ηΔΩ][2π(n₀ sin θ₀)²], wherein θ₀ is anangle of detection direction, n₀ is an optical refractive index of amedium in which said measurement is carried out, wherein ΔΩ is ameasurement solid angle of detection, wherein η is a constant ofproportionality associated with said filter, and wherein v(θ₀), is asignal delivered by said detector of scatter in said direction θ₀ whensaid source illuminates said sample.